1. Technical Field
The present invention relates to a substrate processing method using a laser process or a laser process and an etching process and particularly to a fine processing method. The present invention also relates to a method of manufacturing a micro lens sheet using the processing method and a transmission screen having a micro lens sheet or array (a sheet or array including plural protrusion-shaped micro lenses) obtained using the method. The present invention also relates to a display device having the micro lens sheet, etc. and a projector having the transmission screen.
2. Related Art
Projection screens used for image projection devices such as rear projection televisions, etc. comprise a Fresnel sheet having plural Fresnel prisms for collimating incident light from projection lenses into substantially-collimated light and a micro lens sheet having plural micro lenses for diffusing the substantially-collimated light from the Fresnel lenses. Particularly, the micro lens sheet is formed by die-cutting thermoplastic resins, etc. with a die in which recessed portions are formed at the portions which form the micro lenses. When the die for a micro lens sheet is manufactured, as for the recessed portions which form the micro lenses, it is necessary to form etching holes for forming the recessed portions in a metal film provided on the surface of a substrate which forms the die. The etching holes could be formed using a laser process.
As a conventional laser processing method, there has been known a method of performing the processing while maintaining a difference between a focusing point and a processing point of a laser beam to be 50% or less of the focal depth so as to perform the processing with a high processing accuracy. That is, the height of an XYZ stage on which a processed subject is placed and which moves the processed subject in the X, Y, and Z axis (height) directions correspondingly to advance of the processing, is measured with a laser displacement gauge, the oscillation of a processing laser is stopped when the difference between the focusing point and the processing point becomes 50% or more of the focal length of the processing optical system, and the processing laser is oscillated when the height of the XYZ stage reaches the set position (for example, see Japanese Unexamined Patent Application Publication No. 9-253877 (see [0005], [0012], and FIG. 1)).
The conventional laser processing method described above is suitable for a case of forming holes having a depth of about 200 to 300 μm in a substrate having a thickness of 0.50 to 1.5 mm, for example, with a processing optical system having a focal depth of 30 to 50 μm.
On the contrary, for example, in the case of manufacturing a micro lens sheet to be used for a projection screen having a screen size of 70 inches, a substrate which forms a die for manufacturing the micro lens sheet is large in thickness and area, for example, a width of 1700 mm, a length of 1000 mm, and a thickness of 10 mm. In a substrate of such size, for example, a bend of ±500 μm might be caused, and also a thickness variation might be caused.
In the conventional laser processing method, since the focal depth of the processing optical system is small, there is a problem in that the etching holes (etching-start holes) having a desired shape and pitch could not be formed in a metal film formed on the substrate by adjusting the height of the XYZ stage because of variation in thickness or bend of the substrate. Further, in the conventional laser processing method, since the XYZ stage, the laser displacement gauge, etc. are required, there is a problem in that the apparatuses are complex and expensive.
Accordingly, the present invention is designed to solve the above-mentioned problem, and it is an object of the present invention to provide a processing method and a processing apparatus capable of fine-processing a metal film formed on a substrate, or the metal film and the substrate without influence of variation in thickness or bend of the substrate having a small thickness and a large area. In addition, it is another object of the present invention to provide a method of manufacturing a micro lens sheet using the processing method or the processing apparatus, a micro lens sheet manufactured with the processing method, and a transmission screen for a display device obtained using a shaping die.